In patent literature concerning catalytic dehydrogenation of ethylbenzene to styrene, the interest has almost always been directed towards improving and optimizing the chemical composition in order to achieve ever more satisfactory performances. The improvements are generally obtained by varying the composition as regards the main components or by using different promoters.
Limited attention has been given so far to the geometry of the catalyst.
The importance of the shape can be directly correlated to the pressure used in the processes. Since the dehydrogenation reaction is accompanied by an increase in volume, a pressure reduction facilitates the shifting of the equilibrium towards the products (styrene and hydrogen), with a consequent improvement of the conversion. The possibility of modifying the shape of the catalyst so as to allow operation at a lower pressure (thus also reducing the pressure drop in the catalyst bed) is therefore desirable.
Furthermore, the dehydrogenation reaction is carried out in the presence of steam to reduce the partial pressure of styrene to shift the equilibrium towards the formation of styrene.
In order to solve this problem, two modifications have been adopted as regards the shape:
1) the granule diameter has been increased (to 5 mm) without altering its length. This has solved the problem only to a very limited extent, since a decrease in the pressure drop has indeed been achieved, owing to the reduced bulk density (and therefore owing to an increase in the void fraction), but at the same time the geometric surface exposed to catalysis has decreased. The result of these two contrasting effects has been a reduction in performance. PA1 2) a three- or five-lobed geometric shape has been introduced. A slight improvement has been achieved in this case. However, one should bear in mind that the lobed shape has the drawback that powder forms more easily, since the lobes are weaker fracture points with respect to the solid cylindrical shape. PA1 Fe.sub.2 O.sub.3 =78%; K.sub.2 O=12%; CeO.sub.2 =5%; Mg=2%; WO.sub.3 =0.9%; MoO.sub.3 =2.1% PA1 Fe.sub.2 O.sub.3 =74%; K.sub.2 O=6%; CeO.sub.2 =10%; MgO=4%; WO.sub.3 =6%
Industrially, the process used for catalyst shaping is extrusion molding. It should be noted that this technologically simple process has a very important limitation: specifically, it does not allow to obtain complex geometric shapes, particularly hollow shapes.
As regards composition, catalysts for the dehydrogenation of ethylbenzene to styrene comprise iron oxide, oxides of alkaline or alkaline-earth metals, and other oxides chosen among cerium, molybdenum, tungsten, and chromium oxide.
The life of the catalysts can be improved by adding chromium oxide as a stabilizer. U.S. Pat. No. 3,360,597 discloses catalysts which contain 0.5-5% Cr.sub.2 O.sub.3 next to 80-90% Fe.sub.2 O.sub.3 and 9-18% K.sub.2 CO.sub.3. The catalyst is prepared according to a process which entails the mixing in water of yellow iron oxide, chromium oxide, and potassium carbonate so as to obtain a paste from which the catalyst is obtained in the form of cylindrical granules by extrusion, drying, and calcination.
U.S. Pat. No. 5,023,225 discloses a catalyst for the dehydrogenation of ethylbenzene to styrene which is based on iron oxide, oxides of alkaline or alkaline-earth metals, and cerium, molybdenum, or tungsten oxide, characterized in that the yellow iron oxide is blended with small amounts of chromium oxide prior to molding the catalyst. The molding process is characterized in that the yellow iron oxide blended with chromium oxide is heated to 500-1000.degree. C. to be converted into red iron oxide before mixing the components in the form of a wet paste. Molding is performed by extrusion.